This invention pertains to a woven strap that may be comfortably worn on the leg or arm to drain or wick static electrical charges from the wearer.
Static electricity provides problems in the electronics and other industries, particularly with the advent of integrated circuits and other microelectronic components. Components such as integrated circuits, for instance, may be disabled or destroyed by over-voltages or power density resulting from static electricity. Certain junctions in such circuits can be destroyed by as little as a 50-volt potential, which radically changes the doping structure in their lattices. Power densities resulting from excessive potential and imperfections in circuit layout or structure can vaporize or radically alter the silicon substrate and thus impair or destroy a circuit's performance. Yet a person walking on a carpet on a dry day can accumulate as much as 30,000 volts of potential, and he can triboelectrically generate thousands of volts by simply changing his position in his chair or handling a styrofoam cup.
Such person can inadvertently discharge such static electric potential into the circuit or component by touching it and causing overvoltage or exessive power density. Additionally, the potential in such a person's body can induce a charge in a circuit that can later cause overvoltage or excessive power density when the circuit is subsequently grounded.
More and more frequently, therefore, those in industries in which integrated circuits and other microelectronic components are handled or assembled are taking measures to limit the failure rate of those circuits and components by attempting to keep them as well as their environment at zero electrical potential. Such measures include providing workers and work stations with anti-static carpet, conductive or dissipative grounded desk top work surfaces, hot air ion generators which emit ions to neutralize static charges, and grounding straps to keep workers at zero potential. The term "conductive" herein, and according to its customary usage in the art, means an electrical resistance of between zero and 10.sup.5 ohms. Similarly, "dissipative" means a resistance of between 10.sup.5 and 10.sup.9 ohms, "anti-static" means a resistance of between 10.sup.9 and 10.sup.14 ohms, and "insulative" means a resistance of more than 10.sup.14 ohms.
A grounding strap must have several features in order to perform its grounding function effectively. First, it must ensure that the wearer's skin is electrically connected to ground. This connection is typically accomplished by a conductive surface on the inside of the strap contacting the skin. The conductive surface is electrically connected to a grounding cord which leads from the strap to a grounded electrical connection. If the electrical contacting means on the inside of the strap becomes dirty or fouled by oil, perspiration or hair, the strap may lose its effectiveness. It is therefore important to use a conductive material on the inner surface of the strap that does not easily become dirty or fouled.
Second, comfort is a premium consideration, because if the strap is uncomfortable, the wearer will be tempted to remove it and can thereby cause damage to electrical components on which he is working. A strap that is easily stretchable, that breathes, that is attractive and that poses mimimum inconvenience to the wearer is therefore highly desired.
The situations in which grounding wrist straps are used heightens the importance of their being comfortable so that they are continuously worn and maintain continuous electrical contact with the skin. A person working on microelectronic components or integrated circuits may be completely unaware that he has accumulated minor static electrical charges, and may therefore unknowingly be in a position to disable circuits on which he is working or which he is handling. If his strap is loose or he has removed it, he may be unaware that electrical discharges transmitted from his fingers are disabling these circuits. (A typical person cannot sense a static electrical discharge of less than approximately 3,500 volts.) No one may discover that the circuits have been disabled or damaged until hours, days or weeks later, when the circuits have been placed in components or devices which fail in the field. Removal and repair or replacement of these circuits once in the field is far costlier than avoiding potential failure while the wearer is handling the circuits. Thus, the wearer's employer typically must depend upon the effectiveness of the wrist strap to maintain a lower failure rate of such electronic circuits and components, by maintaining continuous electrical contact with the wearer's wrist and by providing him minimum temptation to remove the strap from his wrist.
These considerations have been addressed by several types of grounding straps. U.S. Pat. No. 4,373,175 issued Feb. 8, 1983 to Mykkanen ("Mykkanen"), for instance, discloses an extensible metal band similar to a Speidel watch band on which a snap fastener for a grounding cord is attached. Such a strap can be uncomfortable, however, and its conductive metal outer surface can prove dangerous to the wearer if it contacts an electrical potential sufficient to electrocute the wearer.
A more comfortable grounding strap is disclosed in U.S. Pat. No. 3,857,397 issued Dec. 31, 1974 to Brosseau ("Brosseau"). Outer and inner conductive polyolefin layers sandwich an intermediate nylon scrim layer to form the band. Hook and loop (Velcro.RTM.) fastening material holds the strap on the wrist. This strap is typical of a number of straps having carbon-suffused synthetics or other conductive polyolefins. Body oil and minerals can accumulate on such surfaces and interfere with electrical contact between the band and the skin. Further, carbon particles tend to wear off onto the wrist, causing black stripes on the wrist. The non-stretchable nature of such bands means that the wearer must adjust them to be tight enough to cause sufficient electrical contact, but loose enough to be comfortable, and skin contact can be lost or intermittent.
Another approach to many of these problems is disclosed in U.S. Pat. No. 4,398,277 issued Aug. 9, 1983 to Christiansen and Westberg ("Christiansen"). This strap is made of knitted stretchable fabric containing stainless steel fibers. A plastic and metal fitting permanently closes the strap into a loop of predetermined size and has a connection for a grounding cord. This strap can prove uncomfortable to the wearer, however, unless his wrist comports with the predetermined strap size offered by the manufacturer. Further, the knitted fabric permits the strap to roll over on itself as it is being pulled over the hand and causes the strap to become thinner as it is stretched. Because the fabric is knitted, it can also "pull" and "run" when snagged. Perhaps more important, it has been discovered that the electrical conductivity of the Christiansen strap decreases as the strap is relaxed, and thus varies from one stretched state to another. This phenomenon probably occurs because the metallic strands in the conductive yarns are pulled more tightly together in the knitted material as it is stretched and are separated from one another to a certain extent in the knitted conductive yarns as the strap is relaxed.